This invention relates to novel red photoluminescent phosphors and to methods for making them. More particularly, this invention relates to novel red-emitting phosphors in combination with electroluminescent devices.
Blue light emitting LEDs emit radiation in the wavelength range of from about 430 to about 480 nanometers (hereinafter nm). When combined with a yellow phosphor, white light of about 6000-8000 K and a color rendering index (CRI) of about 77 is produced.
White light can also be obtained by combining red, green and blue LEDs. However, since different LEDs have different electrical and optical characteristics, e.g., lumen versus lifetime profiles, output light power versus input current curves and resistance versus temperature curves, such combinations fail to deliver light having consistent chromaticity and uniformity. Further, three LEDs require three current regulators, which adds to the cost of making such devices, and the resulting combination is not compact.
Blue LEDs have also been combined with phosphors to convert part of the blue light to red and green light, to produce white light. Suitable phosphors must have a high efficiency of excitation at about 430-480 nm, and a wide chromaticity zone.
Further, phosphors have been combined with other light emitting or electroluminescent devices, such as those using an organic layer that emits light when excited by electrical energy. Phosphors can also be combined with LEDs to convert the color of an LED or other electroluminescent device to a different emission wavelength, as, from blue to red.
Thus suitable red phosphors to combine with electroluminescent devices to change the light emission or chromaticity have been sought.
The present invention is directed to red strontium-calcium sulfide phosphors having the formula
SrxCa1xe2x88x92xS:Eu2+,Y
wherein x is a number of from about 0.3 to 0.8, and Y is one or more halogens, in either their atomic or ionic forms, and to a method for making them. These phosphors have a high quantum efficiency, up to 95%. They are useful to change or convert light from electroluminescent devices to a different emission at various wavelengths.
The host crystal, SrxCa1xe2x88x92xS, is a solid solution in which the ratio Sr:Ca can be changed arbitrarily. The emission spectrum of the material shifts its peak between 605 and 670 nm with changes in the strontium to calcium ratio.
By combining the present red phosphors with green phosphors and blue LEDs, full color white light can be obtained.
These red phosphors are made by
a) forming a mixture of the sulfate salts of strontium and calcium;
b) adding a solution of europium oxide to the sulfate precipitate;
c) firing the solids to a temperature of about 900xc2x0 C. in a forming gas atmosphere;
d) firing to a temperature of about 1000xc2x0 C. in hydrogen sulfide atmosphere to convert the sulfate to the corresponding sulfide;
e) adding an appropriate amount of a halide activator; and
f) firing the mixture of step e) to about 1100xc2x0 C. in nitrogen atmosphere.
The present phosphors can be efficiently excited by the radiation of a blue light LED or other electroluminescent device to produce a red emission, and can be combined with appropriate phosphors to provide full color white light.